In a battery assembly process of a separator for a secondary battery, a hipot test is carried out by applying a high pressure voltage with an aim to detect of a pin-hole of the separator, thickness non-uniformity of the separator, and impurities.
The term ‘hipot’ as used herein is an abbreviation of ‘high-potential’ and is a word being widely known in the art, and is understood as representing a sort of electrical safety testing means used to prove the electrical insulation in electrical products such as electrochemical devices. To conduct a hipot test, a dielectric breakdown test, a dielectric withstanding voltage test, and an insulation resistance test are generally carried out, and a dielectric breakdown voltage in the dielectric breakdown test may be carried out, for example, by measuring a breakdown voltage when dielectric failure occurs.
The conditions of satisfying the hipot test have a trade-off relationship with the requirements for secondary batteries in recent years, that is, a reduction in separator thickness for high capacity, an increase in separator porosity for high rate characteristics, loading of organic particles/inorganic particles and an increase in loading amount for improved thermal properties, and a change requirement of a cathode active material.
More specifically, first, a correlation between the separator thickness and the hipot characteristics is as follows.
It is known that the discharge start voltage follows Paschen's Law as represented by the following equation:V(discharge start voltage)∞P(gas pressure)×D(distance between electrodes)
That is, as the separator thickness reduces, the discharge start voltage increases in proportion to the distance between electrodes.
Subsequently, a correlation between the separator porosity and the hipot characteristics is as follows.
In a hipot test, a medium of a leakage current is gas, and a separator serves as a barrier to prevent it. Thus, as the separator porosity increases, a gas fraction increases but the barrier becomes thinner.
Also, a correlation between a porous coating layer in a separator and the hipot characteristics is as follows.
Under the assumption of no impurities, a separator acts as a dielectric barrier during a hipot test. That is, as a dielectric constant of a separator decreases, a dielectric barrier increases and thereby a discharge start voltage increases. A film-type separator, for example, a separator manufactured using a polyolefin-based film has the same dielectric constant, but a separator with a porous coating layer formed on a porous substrate such as a polyolefin-based film has different dielectric constants due to the presence of the porous coating layer. For example, a dielectric constant of a polyethylene film is 2.25, while a dielectric constant of alumina is 9 or more. Thus, even a separator of the same thickness may have different dielectric breakdown voltages based on whether or not the separator includes a porous coating layer on at least one surface of a porous substrate, and the dielectric breakdown voltage may change based on the density of organic particles/inorganic particles included in the porous coating layer.
Also, recently, the demand for high capacity/high power batteries is gradually increasing in small and medium and large battery markets, and a thin-film separator suitable for high capacity/high power battery design needs to have low electrical resistance and maintain stability.
Such a thin-film separator has poor thermal/mechanical properties, and thus, to enhance the safety, a composite separator is manufactured by applying organic/inorganic heat-resistant coating. However, an organic/inorganic heat-resistant coating layer serves as a resistive layer which blocks a flow of lithium ions and has a negative influence on cycle characteristics. During heat setting, a wet porous separator surface is first exposed to high temperature hot air and absorbs a high quantity of heat, and when a heat setting temperature is high, a change in fibril morphology on the surface is very severe. The two-dimensional and three-dimensional phase morphology changes of the separator pores change from an open pore structure to a closed pore structure, and have significant influence on the lithium ion transfer capability in the cycle characteristics.
Accordingly, it is required to manufacture a wet composite separator which has enough technical performance/competitiveness to satisfy the current unfavorable hipot conditions, and may improve the surface morphology of a porous separator and maintain the resistance at a proper level after organic/inorganic coating, and through this, it needs to improve the cycle characteristics of a high power battery.